I have not actually built my door/cover yet, but...
I did some research on this and found that very few materials are transparent to the 10.6um wavelength of CO2 lasers. I found this document (
http://www.synrad.com/LaserFacts/laser%20safety%20requirements.pdf) document at Synrad that agrees with that. I added some info to my research section (h
ttp://www.buildlog.net/cnc_laser/cnc_build_research.htm#SafetyWindow).
Since I did that research I have been cutting and engraving a lot of materials. You can engrave all day on a piece glass, acrylic or polycarbonate and no real energy appears to go through. As an experiment I put a piece of glass on top of a white piece of paper and engraved very stongly until I enventually shattered the glass. No discoloration appeared on the paper.
With that said, I now realize the invisible IR is only half the story. When cutting or engraving most materials, the point where the laser strikes the material is an intense white light, like the tip of a welding torch. This must be burning the material and gases. It hurts your eyes to look at it even through IR laser safe glasses. Therefore, you really want something to shield both the primary IR light as well as the intense visible light.
This may be why some lasers have tinted covers. I am almost temped to buy some flip up retro looking welding googles.